<!DOCTYPE html>
<html>
<head>
<style type="text/css">
#separator, #clear {
 height: 20px;
 background-color: green;
}
#parent {
 margin-top: 20px;
 background-color: lime;
}
#float {
 width: 100px; height: 100px;
 background-color: blue;
 margin-right: auto;
}
</style>
</head>
<body>
<div id="separator"></div>
<div id="parent">
 <div id="float"></div>
 <div id="clear"></div>
</div>
</body>
</html>
